Agrochemicals are typically applied either as a solution or as a suspension of a fine powder. It is often desirable for the agrochemical to remain either near the surface of the soil (in the case of many insecticides and pre-emergent herbicides, for example) or within the root zone for active agents that are taken up through the roots, such as fertilizers and certain herbicides. However, in many cases, agrochemicals are rapidly depleted from the soil zone in which they are most effective. Among the mechanisms of depletion are metabolism by bacteria, surface runoff and wash-down deep into the soil by rain, and volatilization. Such depletion leads to a loss of efficacy, and can also result in contamination of surface and groundwater.
One approach to extending residual activity and reducing the offsite movement of an agrochemical involves impregnating the agrochemical into an inert matrix. Under favorable conditions, controlled release of the agrochemical can take place. For example, U.S. Pat. No. 6,890,888 describes impregnating urea and other fertilizers into expanded perlite, which can be soil-applied to achieve controlled release. Agrochemicals can also be impregnated into clays or polymer particles, as described, for example, in U.S. Pat. No. 5,908,632 and the references cited therein. Alternatively, an agrochemical can be chemically linked to a polymer. For example, Kenawy et al. (J. Appl. Polymer Sci. 80: 415-21 (2001)) describes linking 2,4-dichlorophenoxyacetic acid (2,4-D) to a polymer backbone via an amide linkage.
For herbicides that can cause crop injury at high rate, micro-encapsulation can reduce crop injury by providing controlled release while reducing off-site movement. For example, Bollich et al. (Weed Technology 14:89-93 (2000)) describes micro-encapsulation of clomazone. Several commercial microencapsulated herbicides are also available, for example, the COMMAND® (FMC Corp, clomazone) and WARRANT® (Monsanto, acetochlor) products.
Achieving controlled release is particularly challenging for agrochemicals that contain carboxylic acid groups. Such agrochemicals are referred to herein as “carboxylic acid agrochemicals.” Carboxylic acid agrochemicals exist in the form of salts or zwitterions when released in the field, rendering them water soluble. Waterborne movement of agrochemicals containing carboxylic acid groups is therefore facile. In addition, the water solubility of these compounds leads to rapid leaching from matrices which can be used for controlled release of other molecules and complicates formation of microcapsules, a process which is typically conducted in a 2-phase, water-organic mixture with the active in the organic phase.
Alkyl esters of carboxylic acid agrochemicals exhibit reduced water solubility. For example, as described in the Herbicide Handbook (9th ed., 2007), the methyl ester of diclofop, the ethyl esters of fenoxaprop-P and desmedipham, and the butyl ester of cyhalofop along with many alkyl esters of 2,4-D are used as herbicides. However, alkyl esters of certain carboxylic acid agrochemicals hydrolyze rapidly in the soil, rendering them more susceptible to microbial degradation. As a result, alkyl esters of such carboxylic acid agrochemicals seldom if ever have significant residual activity. On the other hand, hindered aromatic esters previously known in the art typically hydrolyze far too slowly and are not practical for controlled release of agrochemicals.
Thus, there exists a need in the art for a method of achieving controlled release of carboxylic acid agrochemicals. This need is particularly acute for molecules which can cause damage to crops in neighboring fields by volatilization, for example, the auxin-mimic herbicides dicamba and 2,4-D.